Diisocyanates which are liquid at room temperature have numerous advantages over solid diisocyanates. Diisocyanates which are liquid at room temperature, such as toluene diisocyanate or hexamethylene diisocyanate, are, as a rule, physiologically harmful because of their high vapor pressure. For this reason, various attempts have been made to start with diisocyanates that are solid at room temperature and convert these into the liquid form by certain measures.
The most commercially important diisocyanates which are solid at room temperature are 4,4'-diphenylmethane diisocyanate and the 2,4'-isomer thereof, which melt at 39.degree. C. and 34.5.degree. C., respectively.
Numerous patents have issued relating to the liquification of diphenylmethane diisocyanate. See, for example, U.S. Pat. Nos. 3,152,162; 3,384,653; 3,394,165; 3,449,256; 3,640,966; 3,641,093; 3,674,828; 3,701,796; 3,883,571; 4,014,935; 4,055,548; 4,088,665; 4,031,026; 4,102,833; 4,115,429; 4,118,411; 4,154,752; 4,177,205; 4,229,347; 4,261,852; 4,321,333; and 4,332,742.
One of the more commercially successful liquid diphenylmethane diisocyanate compositions is produced according to U.S. Pat. No. 3,644,457. These isocyanates are prepared by reacting 4,4'- and/or 2,4'-diphenylmethane diisocyanate with a branched aliphatic dihydroxy compound to produce a product which is liquid at room temperature. According to this patent, 1 mol of a diphenylmethane diisocyanate is reacted with from about 0.1 to about 0.3 mols of a poly-1,2-propylene ether glycol having a molecular weight of from 134 to 700.
Although the isocyanates made according to U.S. Pat. No. 3,644,457 have met with significant commercial success, such isocyanates are generally linear in structure (i.e., having NCO functionalities of 2). For many applications, it would be desirable to provide liquid diphenylmethane diisocyanate compositions having some branching (i.e., having NCO functionalities of greater than 2). Additionally, for some applications it would be desirable to incorporate some unsaturation in the isocyanate composition.
It is an object of the present invention to provide improved organic isocyanates which are liquid at room temperature and which remain liquid even on prolonged storage down to 15.degree. C. Another object of this invention is to provide an improved process for preparing such liquid organic isocyanates. Finally, it is an object of this invention to provide a liquid isocyanate which has some branching and which may have some incorporated unsaturation.